The present invention relates to a material for packaging light-sensitive materials which deteriorate on exposure to light, in particular, to a material for packaging light-sensitive materials for photographic use such as silver halide photosensitive materials for monochrome and multi-color photographic films, printing films and X-ray films, diazo photosensitive materials, or the like. Materials for packaging light-sensitive materials should possess good light-shielding properties, physical strengths (breaking strength, tear strength, resistance to impact perforation, moistureproofness, heat seal properties (heat seal strength, heat seal properties at low temperatures and hot seal strength), and antistatic properties.
It is difficult for a single film material to possess all these properties. Accordingly composite laminate films comprising a low density polyethylene film in which carbon black or pigment powder is incorporated and a flexible sheet such as paper, aluminum foil, and cellophane have been used as a material for packaging light-sensitive materials.
As the light-shielding paper for such packaging materials, for example as a backing sheet for photographic roll film, a composite sheet comprising a flexible substrate (a so-called "flexible sheet") such as paper coated with a thermoplastic resin layer has widely been used.
A wide variety of resins have been used for the thermoplastic resin layer, all having both merits and demerits.
For instance, low density polyethylene (hereinafter referred to as "LDPE") when used for thermoplastic resin layer has the following demerits: various kinds of defects due to its insufficient flexibility tend to develop; heat seal temperature is high; no heat-sealability with a polystyrene spool for photographic film is obtained; when LDPE is heat-sealed with LDPE, heat-seal defects due to dusts, powders etc. (i.e. when powdery adhering contaminants are present, heat-sealability is decreased) often occur; addition of a light-shielding material decreases strength of film and sealing strength.
Ethylene-vinyl acetate copolymer (hereinafter referred to as "EVA") has drawbacks in which its moisture absorbability is too high; separation from a cooling roller cannot be readily achieved, thereby resulting in low speed of the extrusion, and application of the melt extrusion on a support or substrate is impossible since separation from the cooling roller having a smooth surface is difficult.
Accordingly, even if EVA might be used with a cooling roller having a surface roughed with protrusions and recesses to provide good separation, blocking of EVA readily takes place and slip properties become worse resulting in poor adaptability to the extruding machine, when the cooling roller having EVA wound thereon is left as it is for several days. Thus prevention of blocking and improvement in slip properties should be carried out by applying powder of cornstarch onto an extruded film. However the powder will give rise to dots and nonuniform colors in the photographic materials. Furthermore, EVA has various defects such as high neck-in and poor cutting readiness or the like. Accordingly products which have been melt-extruded and coated with merely EVA have not been used as a material for packaging light-sensitive materials. In particular, EVA including not less than 10 mol % of vinyl acetate (VA) which is heat-sealable with polystyrene has been difficult to practically use since it to a great extent suffers from above-mentioned defects.
Melt-extrusion coating of acrylic ester type copolymer such as ethylene-ethylacrylate copolymer (hereinafter referred to as "EEA") as is disclosed in U.S. Pat. No. 3,832,218 issued on Aug. 27, 1974 cannot overcome the defects similar to those of the EVA coating although the acrylic ester-coated product exhibits excellent flexibility, heat seal properties at low temperatures and heat seal properties in the presence of adhering contaminants. Besides, light-shielding materials such as carbon black will not be uniformly dispersed since finely divided gel particles are difficult to be removed. Accordingly it has been necessary to make a single layer thick to provide a sufficient light-shielding ability by the single layer. Still more, acrylic ester copolymer also readily involves blocking similar to EVA. Accordingly, the melt extrusion coated product with acrylic ester copolymer has not hitherto been put into practical use as a packaging material for light-sensitive materials.
Conventional approaches to lamination of a single layer of the thermoplastic resin onto a substrate have many problems as described hereinabove. Therefore, light-shielding paper which is one of materials for packaging light-sensitive materials has hitherto been produced by applying on a substrate a solution of EVA dissolved with a solvent such as toluene, to the solution being added no less than 20% of additives such as carbon, etc. This process is referred to as solution coating process which results in a high manufacturing cost.